Arhin, Samuel Gyebi (2024) Waste‑Based Chain Elongation for Medium Chain Fatty Acids Production. [Tesi di dottorato]

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Abstract

The sustainable conversion of organic waste streams into biochemicals such as medium-chain fatty acids (MCFAs) represents a crucial node in the development of a circular economy and averting the negative impacts of unsustainable sources. The overarching goal of this PhD thesis was to investigate the valorization of food waste via chain elongation to generate MCFAs and optimize the process via the addition of another organic waste stream, namely lignocellulosic materials. In the first place, acidogenic fermentation experiments were conducted to optimize the synthesis of electron acceptors and donors for the subsequent chain elongation phase. The pH, temperature, and food-to-microorganisms (F/M) ratio were identified as critical process lever for gearing the biochemical reactions in the acidogenic fermentation phase towards the desired intermediates for maximizing MCFAs production in the chain elongation phase. By manipulating these parameters, up to 719 ± 94 mgCOD/gVS of volatile fatty acids with desirable carbon chain lengths and 684 ± 34 mgCOD/gVS of lactic acid could be synthesized as electron acceptors and donor, respectively. These intermediates from the acidogenic fermentation phase were then used as co-substrates with carbon monoxide (CO) and ethanol. Co-fermentation of CO and the acidogenic fermentation products improved MCFAs. CO at a partial pressure of 0.25 bar completely inhibited methanogenesis and availed more carbon for MCFA production, unveiling a sustainable chain elongation route without the participation of bioactive methanogenic inhibitors such as 2-bromoethanesulfonate. In the subsequent experiment, the potential co-operative effect of co-fermenting lactic acid derived from food waste acidogenic fermentation and ethanol on MCFAs production was assessed. At the optimum ratio, lactic acid and ethanol synergized to facilitate MCFAs production. Moreover, by capitalizing on the respective net proton consuming and net proton releasing behaviours of the lactate and ethanol-driven chain elongation, an in-situ buffering capability was activated to circumvent the need for external chemicals for pH control. A key bottleneck found during chain elongation was the poor response of the microbiome to high CO partial pressures. Further research should focus on enhancing the resilience of the microbiome to high CO partial pressures.

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